The invention relates generally to a system for cracking hydrocarbon molecules. More particularly, the invention provides a fluidized catalytic cracking system having a riser equipped with contoured ribs. The contoured ribs enhance mixing of the catalyst and the hydrocarbon feedstock that flow through the riser.
The most common method for catalytic cracking presently in use in the oil refining industry is fluidized catalytic cracking (FCC). The FCC process is utilized to crack hydrocarbon materials such as oil. Cracking refers to the conversion of petroleum fractions having a high boiling point into products having a lower boiling point. The cracking process is usually performed in a vertically-oriented conduit, or riser, that forms part of an FCC system. Typically, hot catalyst particles in an aerated (fluidized) state are introduced into a bottom portion of the riser, and are induced to flow upward. A hydrocarbon feedstock is injected into the catalyst flow as the catalyst travels through the riser. The hydrocarbon feedstock, in general, is significantly cooler than the catalyst and rapidly vaporizes upon contact with the catalyst.
Optimal cracking conditions in an FCC process require a substantially immediate and homogenous mixing of the catalyst and the hydrocarbon feedstock. Such mixing is difficult to achieve, however, and stratified regions of hot catalyst and cold hydrocarbon feedstock typically appear within the catalyst-hydrocarbon flow. Over-cracking and thermal cracking of the hydrocarbon molecules typically occur in the catalyst-rich areas of the flow. Conversely, incomplete cracking of the hydrocarbon molecules usually occurs in hydrocarbon-rich flow regions. These factors can substantially reduce the overall yield of the FCC process. In addition, over-cracking, thermal cracking, and incomplete cracking have undesirable side-effects such as deactivation of the catalyst within the riser due to coke laydown, regeneration of the catalyst within the regenerator due to the combustion of air and residual coke, and the production of excessive amounts of lower-boiling-range gaseous reaction products, e.g., propane and butane gases. Hence, effective methods for mixing the catalyst and the hydrocarbon feedstock within the riser are critical to the cracking process.
Radial-feed-injection of hydrocarbon feedstock is a commonly-used technique for improving the mixing of catalyst and hydrocarbon feedstock in FCC systems. This technique involves the use of radial-feed atomizing nozzles positioned around the circumference of the riser. Steam is typically directed to the nozzles to assist in the atomization of the hydrocarbon feedstock. Radial-feed nozzles, in general, produce a more uniform spray pattern of hydrocarbon feedstock than other injection techniques. Common radial-feed nozzles form a flat, fan-shape spray jet that diverges at an angle within the range of approximately 40 to 65 degrees after leaving the atomizing nozzle (this angle is known as the xe2x80x9cspray anglexe2x80x9d of the nozzle). Spray angles above approximately 65 degrees increase the potential for the erosive spray jet to impinge on the inner surface of the riser.
One of the drawbacks of radial-feed injection is the presence of gaps in the hydrocarbon spray pattern. In particular, the angled profile of the individual spray jets produces gaps in the spray pattern proximate the inner surface of the riser. This phenomenon is illustrated in FIG. 8. FIG. 8 is a cross-sectional view of a common riser 200 having an inner surface 200a. A plurality of radial-feed atomizing nozzles 202 are positioned around the circumference of the inner surface 202a. The nozzles each produce an individual spray jet 204a of hydrocarbon feedstock. The spray jets 204a collectively form a spray pattern 204 within the riser 200.
A plurality of gaps 204b appear in the spray pattern 204, as shown in FIG. 3. The gaps 204b are a result of the fan-shaped profiles of the jets 204a, and the plug-flow nature of the catalyst as it travels upward through the spray region of the riser 200. The combined area of the gaps 204b can be as large as fifty percent or more of the cross-sectional area of the riser 200. Furthermore, the unmixed region of catalyst downstream of the gaps 204b can persist for twenty-five feet or more in the riser of a typical FCC system.
Various techniques for improving the mixing of catalyst and hydrocarbon feedstock in FCC systems have been developed. For example, the use of venturi tubes, draft tubes, and vortex mixing to disturb the catalyst flow near the point of injection of the hydrocarbon feedstock has been described in U.S. Pat. Nos. 4,523,987; 5,622,677; 4,578,183; and 5,318,691. Other mixing techniques include increasing the normally-occurring turbulence within the catalyst-hydrocarbon flow through the use of turbulence tips (U.S. Pat. No. 4,753,780) and kick-off rings (U.S. Pat. No. 5,851,380) affixed to the inner surface of the riser. Various other configurations, including feed-injection cone assemblies (U.S. Pat. No. 5,554,341) and arcuate mixing elements (European Pat. No. 832,956) have also been proposed. Furthermore, non-standard injection arrangements have been suggested in U.S. Pat. Nos. 5,139,748; 4,883,583; and 5,348,644, and in European Pat. No. 911,379.
The above-noted mixing techniques have not proven entirely effective in eliminating spray-pattern gaps such as the gaps 204b. Hence, a need currently exists for an FCC system comprising a riser having geometric features that eliminate such gaps, thereby achieving more effective mixing of catalyst and hydrocarbon feedstock within the riser.
An object of the present invention is to provide a fluidized catalytic cracking (FCC) system that effectively mixes a flow of catalyst and hydrocarbon feedstock. In accordance with the this object, a presently-preferred embodiment of the invention provides an FCC system comprising a riser. The riser has an outer surface and an inner surface. The inner surface defines a central passage that extends substantially along a longitudinal axis of the riser. The central passage is used to transport a hydrocarbon feedstock and a catalytic material.
The inner surface of the riser has an elongated rib disposed thereon for mixing the hydrocarbon feedstock and the catalytic material. The rib has an inner wall that faces the central passage. The rib has a thickness defined by a distance between the inner wall and the outer surface of the riser. The thickness of the rib varies along at least a portion of a length of the rib.
According to another aspect of the invention, a preferred embodiment of an FCC system comprises a riser having an outer surface and an inner surface. The inner surface defines a central passage extending substantially along a longitudinal axis of the riser. The FCC system also includes a plurality of atomizing nozzles coupled to the riser. The atomizing nozzles are adapted to inject a hydrocarbon feedstock into a flow of catalytic material within the central passage. The atomizing nozzles thereby form a spray pattern of the hydrocarbon feedstock within the central passage. The FCC system further comprises a plurality of elongated ribs disposed along the riser inner surface.
In accordance with a further aspect of the invention, a preferred embodiment of an FCC system comprises a riser having an outer surface and an inner surface. The inner surface defines a central passage extending substantially along a longitudinal axis of the riser. The central passage is used to transport a hydrocarbon feedstock and a catalytic material.
The FCC system further comprises an elongated rib disposed along the riser inner surface. The rib is used to mix the hydrocarbon feedstock and the catalytic material. The rib is contoured so that a radial distance between the rib and the longitudinal axis of the riser varies along at least a portion of a length of the rib.